Soda ash or sodium carbonate is an inorganic salt made from the mineral trona. Soda ash is one of the largest volume alkali commodities made in the United States. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
Large deposits of the mineral trona in southwestern Wyoming near the Green River Basin have been mechanically mined since the late 1940's. In 2007, trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production. Trona ore is a mineral that contains about 85-95% sodium sesquicarbonate dihydrate (Na2CO3.NaHCO3.2H2O). Trona ore contains insoluble matter in the form of shale. The shale contains various constituents such as organic kerogeneous matter (e.g., 0.1-1% as carbon) and dolomitic and silica bearing materials (e.g., about 5-15%), such as dolomite, quartz, feldspar, clay.
The crude trona is normally purified to remove or reduce impurities, primarily shale and other water insoluble materials, before its valuable sodium content can be sold commercially as: soda ash (Na2CO3), sodium bicarbonate (NaHCO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), sodium sulfite (Na2SO3), a sodium phosphate (Na5P3O10), or other sodium-containing chemicals.
To recover these valuable alkali products, the ‘monohydrate’ commercial process is frequently used to produce soda ash from trona. In the production of soda ash, crushed trona ore is calcined (e.g., heated) to decompose the sodium sesquicarbonate to sodium carbonate.2Na2CO3.NaHCO3.2H2O→3Na2CO3+5H2O(g)+CO2.(g)
The calcination drives off water of crystallization and forms crude soda ash. During calcination, a part of the water insoluble silicate bearing material contained in the ore is converted to soluble silicates. The calcined ore is dissolved in water or dilute sodium carbonate liquor to give a saturated solution of ˜30% Na2CO3 (depending upon the temperature of the solution) containing soluble impurities. The soluble impurities may comprise silicates, organics, chlorides, and sulfates. The insoluble material is separated from the resulting saturated solution. This clear sodium carbonate-containing solution is fed to an evaporative crystallizer. As this solution is heated, evaporation of water takes place effecting the crystallization of sodium carbonate into sodium carbonate monohydrate crystals (Na2CO3.H2O). The monohydrate crystals are removed from the mother liquor and then dried to convert it to anhydrous soda ash (Na2CO3). The mother liquor is recycled back through a crystallizer circuit for further processing into sodium carbonate monohydrate crystals.
The crystallization step however concentrates impurities in the mother liquor. Indeed, by the effect of water evaporation, the soluble impurities such as organics, silicate, chloride and sulfate, become concentrated in the crystallizer. If this is allowed to continue, eventually the concentration of the impurities builds to a point where the resulting sodium carbonate product quality is negatively impacted. To avoid contamination and deterioration of crystal shape and hardness by the impurities and to prevent the buildup of these impurities in the crystallizer, a portion of the crystallizer liquor must be purged. This can result in a loss of up to about 10% of the soda values. The purge liquor includes soda ash as well as impurities, such as organics, sodium bicarbonate, sodium chloride, sodium sulfate, and sodium silicate. This purge liquor typically contains ca. 23-28% sodium carbonate, 1-4% sodium bicarbonate and 0.2-1% silicate.
In the manufacture of soda ash, a system of storage ponds has been used to accommodate disposal of the effluent streams including mine water, and other sources of waste waters inherent to the process. The purge liquor exiting the crystallizer is typically stored in one or more tailings (waste) ponds which use up large areas of land. In the tailings pond, this purge liquor evaporates and/or cools resulting in the crystallization of sodium carbonate decahydrate contaminated with varying amounts of these impurities, including silicates and organics, the impurities amount in the decahydrate being albeit smaller than the pond solution. This ‘deca’ deposit on the pond bottom reduces the total pond volume. If this solid mass is not removed, it eventually fills the available pond volume until an increase in pond volume must occur, by raising existing dikes, expanding the existing pond, or constructing a new pond. It would be beneficial to collect and use the deposited sodium carbonate decahydrate mass from the tailings pond, as the removal of this solid mass would free up previously filled volume in the tailings pond. Since the recovered ‘deca’ mass contains valuable sodium carbonate content that would otherwise have to be mined, it would be beneficial to recycle the collected ‘deca’ to the monohydrate crystallizer for the purpose of recovering the soda values.
Despite the withdrawal of a portion of the crystallizer liquor as a preventative measure to prevent accumulation of impurities in the crystallizer, the presence of water-soluble silicates in the crystallizer liquor causes severe scaling of the surfaces of equipment in which this saturated solution is handled, for examples lines, tanks, pumps, and particularly the crystallizer heat-exchanger which handles the liquor in a recycling loop connected to the crystallizer. A scale buildup containing silicates generally formed on exposed surfaces of the crystallizer heat exchanger requires frequent and expensive high pressure washes.
Additionally, Applicants have found that the presence of water-soluble silicates in the crystallizer liquor seems to impact the morphology of crystallization, and to render the resulting crystalline sodium carbonate product more friable. Although not desiring to be bound by a theoretical explanation, it is believed that the propensity of small particles formation, also called ‘fines’, in the final product is caused, at least in part, to the silicate impurities present in the liquor during crystallization. Since there is a specification on particle size distribution on the final crystalline sodium carbonate product, the greater fraction of these fines in the product size distribution diminishes the yield of salable product, and these fines have to be recycled to the process to prevent significant loss in soda values.
Moreover, Applicants have further found that the propensity of fines formation is even greater when a reclaimed solid which comprises sodium carbonate decahydrate is recycled to a sodium carbonate-containing liquor which is ultimately fed to the crystallizer. It is believed that the product degradation is due to the higher amount of silicates being carried over in the liquor from the reclaimed solid which comprises sodium carbonate decahydrate. For example, it has been found that while a calcined trona liquor may contain about 70 ppm silicon, a reclaimed crystalline sodium carbonate decahydrate solid may contain about 600 ppm silicon. When 10% of the liquor is made from dissolved reclaimed sodium carbonate decahydrate solid with the remainder being the dissolved calcined trona, there is almost a doubling in the ppm silicon level in the resulting solution. Consequently, the impact of silicate impurities is even more felt when a reclaimed solid containing sodium carbonate decahydrate is recycled in the soda ash plant for further crystallization.
Applicants have further observed that propensity of foam formation in the sodium carbonate monohydrate crystallizer is an additional operational issue which is even more pronounced when the reclaimed solid comprising sodium carbonate decahydrate recovered from a tailings pond is recycled to the soda ash process to form the crystallizer feed with dissolved calcined trona. It is suspected that the greater foam incidence with this recycle is due to foam-causing agent(s) in the crystallizer feed which are carried over from the recycled reclaimed solid.
In addition to the soda ash production process, other processes utilizing saturated or near-saturated sodium carbonate-containing solutions (e.g., derived from calcined trona) as feedstocks may be impacted by the presence of soluble silicates in such solution, particularly if a portion of such solution is derived from a dissolved reclaimed solid such as a reclaimed solid comprising sodium carbonate decahydrate. Examples of such processes include a sodium sulfite production process which may use a sodium carbonate-containing solution as feedstock to the sulfite reactor, and/or a sodium bicarbonate production process which may use a sodium carbonate-containing solution to serve as feedstock to the bicarbonate reactor. Such process includes forming sodium sulfite or bicarbonate by reaction of a sodium carbonate-containing solution with sulfur dioxide or carbon dioxide gas. Sodium sulfite crystals are typically formed in a sulfite crystallizer, while the sodium bicarbonate crystals are typically formed in the bicarbonate reactor at the same time as the reaction takes place.
Since the sodium carbonate feedstock contains silicate impurities, such silicates concentrate and precipitate in the sodium sulfite or bicarbonate process which would negatively impact the final product quality. For example, there are quality specifications limiting water insoluble matter in photo-grade sodium sulfite imposed by ISO 418 Photography—Processing chemicals—Specifications for anhydrous sodium sulfite. The removal of the silicon-containing impurities before they can contaminate the final crystalline sodium sulfite product would allow the sodium sulfite process which uses dissolved calcined trona and/or dissolved reclaimed solid (such as comprising sodium carbonate decahydrate) as sodium carbonate feedstock(s) to make a photo-grade sodium sulfite.
An existing remedy for decreasing silicate levels in the sodium sulfite process includes cooling a purge stream withdrawn from the crystallizer through cooling units followed by a filtration step to decrease the content in precipitated silicate. Such cooling units can be used to cool the reactor and to decrease the silicate levels when making photo-grade sodium sulfite. As a result, production is limited by the reactor temperature resulting in decreased overall production rates. In addition, this remedy for silicate removal is carried out after crystallization is performed, and as such, there is a buildup of silicate scale on the crystallizer heat exchanger (which is connected in a recirculation loop to the crystallizer).
It is thus apparent that a need exists for a more effective method for reusing a waste in a process to form a final product comprising sodium carbonate, bicarbonate or sulfite, but wherein the waste contains impurities which may cause a negative impact on product quality and/or on operation of the process. A need exists for obtaining a less-friable crystalline anhydrous sodium carbonate product or a photo-grade crystalline sodium sulfite or a bicarbonate product with a reduced silicate content from an impure feedstock solution which comprises water-soluble impurities (e.g., silicates, organics) derived from trona ore and/or from a reclaimed solid or liquor waste. There is also a need to minimize operating costs by reducing downtime for maintenance of equipment which is exposed to silicates (due to cleaning of scale). There is also a need to minimize loss of soda values by recycling a crystalline sodium-containing reclaimed solid recovered from a waste pond and/or a waste liquor carrying silicates and organic impurities, yet without impacting the quality and yield of salable soda ash product or the quality of other products (e.g., sodium sulfite, sodium bicarbonate) which are made from sodium carbonate-containing solutions, particularly those comprising the recycled waste. There is also a need to reduce operational issues (such as scaling of equipment, foaming in crystallizer) created at least in part from the reintroduction of impurities such as water-soluble silicates and/or organics when recycling a reclaimed solid recovered from a tailings pond or a secondary crystallizer.